This invention relates to an apparatus for controlling the yawing behavior of an automotive vehicle supported on a pair of front road wheels and a pair of rear road wheels.
In general, drivers turn the vehicle in a circle with a feeling of vehicle position and direction, that is, vehicle slip angle (normally referred to as yaw angle). However, a satisfactory sensor for measuring vehicle slip angle is not available at this time. Because of this, it is the current practice to control the vehicle behavior by controlling the vehicle yaw rate to a target value, as disclosed in Japanese Patent Kokai Nos. 63-312271 and 4-334660. With such a conventional vehicle yawing behavior control, however, the vehicle behavior would be controlled in a direction to promote unstable vehicle behavior upon occurrence of vehicle lateral slip. In order to solve this problem, feedback control may be required not only for the vehicle yaw rate, but also for the vehicle slip angle. The vehicle slip angle feedback control is made, according to a deviation of the actual vehicle slip angle from a reference slip angle, to ensure that the actual vehicle slip angle is correct to maintain the reference slip angle. In this case, the yawing motion value M to be controlled is given as: EQU M=K1[(d/dt).phi.-(d/dt).phi.*]+K2(Sa-Sa*)
where (d/dt).phi. is the actual vehicle yaw rate, (d/dt).phi.* is the target vehicle yaw rate, Sa is the actual vehicle slip angle, Sa* is the reference slip angle, and K1 and K2 are control constants. The term K1 [(d/dt).phi.-(d/dt).phi.*] corresponds to the value Mb obtained by the feedback control of the vehicle yaw rate and the term K2 (Sa-Sa*) corresponds to a value Ma obtained by the feedback control of the vehicle slip angle. The value Mb acts to produce a yawing motion inside with respect to the vehicle turning movement, and the value Ma acts to produce a yawing motion outside with respect to the vehicle turning movement. That is, the values Ma and Mb act in the opposite directions to cancel each other. For this reason, the conventional vehicle behavior control cannot stabilize the vehicle behavior to a sufficient extent.